Cardiovascular disease is today considered the main cause for mortality in the Western world. Such cardiovascular diseases include atherosclerosis in which plaque builds up on the inside of arteries. Plaque is made up of fat, cholesterol, calcium and other substances found in the blood. Over time, the plaque hardens and narrows the arteries reducing the flow of oxygen-rich blood to organs and other parts of the body. This can lead to serious problems, including heart attack, stroke or even death.
Atherosclerosis can affect any artery in the body, including arteries in the heart, brain, arms, legs and pelvis. As a result, different diseases may develop based on which arteries are affected. When plaque builds up in the coronary arteries reducing or blocking blood flow to the heart, it can lead to chest pain and heart attack. This is referred to as coronary artery disease (CAD), also called heart disease, and it is currently the leading cause of death in the United States. Carotid artery disease occurs when plaque builds up in the carotid arteries that supply oxygen-rich blood to the brain. When blood flow to the brain is reduced or blocked, it can lead to a stroke. Peripheral arterial disease occurs when plaque builds up in the major arteries that supply oxygen-rich blood to the legs, arms and pelvis. When blood flow to these parts of the body is reduced or blocked, it can lead to numbness, pain and, sometimes, dangerous infections.
A common treatment for atherosclerosis comprises insertion of a stent into the narrowed blood vessel via angioplasty to prevent or counteract the localized flow constriction. Stents are typically made of metal mesh, e.g. stainless steel. After expansion by balloon angioplasty with a stent, plaque is pushed back and the artery wall stretches allowing increased blood flow. The stent prevents the stretched blood vessel from reverting to its initial size.
Unfortunately, as a result of neo-intimal tissue growth, in-stent restenosis may occur narrowing the treated blood vessel over time. In-stent restenosis is defined as greater than 50% stenosis and is estimated to occur up to 30% in bare metal stents and 7%-11% in drug eluding stents.
Existing protocols for detecting and determining extent of in-stent restenosis include catheterization, and imaging. However, catheterization is an invasive process with a high rate of complications, and existing non-invasive diagnostic protocols only provide indirect measurement. Accordingly, post stenting follow-up is significantly impaired, being neither sensitive nor specific enough, and not fully safe.
A need thus exists for a non-invasive tool for diagnosing and monitoring in-stent restenosis. More particularly, there is a need to precisely determine and track the location of a stent within a living body and then directly measure vascular flow within the stent.
More generally, a need exists for a non-invasive tool for in-vivo localization and diagnosis of metallic objects, such as artificial heart valves, orthopedic implants and screws, metallic shrapnel, and the like.